In the last instalment, I described the effects of the rotation of the Earth on how we see the stars above us (and on which stars we see and which ones we don't). That's all very well, but there's one other major type of movement of our planet that affects, and much, the way the sky looks like: we also orbit our Sun, and complete one orbit in one year.
The orbit of the Earth around the Sun has one major, immediate effect: the position of the Sun on the sky (against the background of stars) changes as the year goes by; the Sun will complete a full circle of the sky in a year, which means that its position changes by about one degree a day (360 degrees in a circle, 365 days in a year) — one degree is about twice the apparent diameter of a full moon. Of course, this is not directly visible: we can't see the stars around the Sun during the day. What we will notice is that the field of stars that is visible at night will subtly change from day to day — the whole field will seem to move westwards by about one degree a day.
One important thing to notice is that this apparent movement of the Sun relative to the stars (or of the stars relative to the Sun) is not parallel to the Equator: it is, in fact, tilted by approximately 23.5° (the exact angle changes over time; nowadays it's more like 23.44°, or 23°26'). This tilt has one very significant effect on Earth, but that's the subject of our next article: the four seasons.
The apparent path of the Sun on the sky has a special name: it's called the ecliptic. This is also the path followed (with varying degrees of precision) by the other planets in their path on the sky, as we'll see in the future. The ecliptic goes through several constellations — to be more precise, twelve. These are called the zodiacal constellations, or the constellations of the zodiac (from the Greek meaning "circle of animals" — most zodiacal constellations are represented by animals). One can see how twelve constellations resulted not only in the twelve astrological signs, but also in the twelve months of the year.
There are four special points on the ecliptic that are worth mentioning:
- the solstices: these are the points of maximum north or south drift (or declination) of the Sun; the northern solstice happens in June, while the southern solstice happens in December
- the equinoxes: these are the points of minimum declination, that is, where the Sun crosses the celestial Equator; they are called the autumnal equinox (which happens in September), when the Sun crosses the Equator moving northward, and the vernal equinox (in March), moving southward.
And that's it for today. As I mentioned, in the next article we'll explore in more details the effects of the tilt in our orbit and the significance of the solstices and equinoxes.